Be careful parking anywhere in Knaresborough the parking wardens are very very keen.
There is a big carpark behind Eastgate surgery that is pay and display but not sure if you could overnight there.
There will not be anything town centre near the castle.
Sorry I cant help any more. I know of a place in Harrogate you could overnight but thats 3 miles away. Cant you get in on any of the local caravan sites. Spen House at Minskip is a CS. Its 6 miles from knaresborough, about 10 min drive. J x
O we have a site we love called the Cud Life near Killinghall, but not sure I would be able to park a 25ft van near the Castle for the evening either. All new to me.
Another idea. I think Shaws trailer park on Knaresborough road at Starbeck take tourers. There is a bus every 10 mins down into knaresborough until about 7pm then about every 20-30 mins until midnight, its about a ten minute journey.
Personally I wouldnt stay overnight on the roadside because that parking warden probably works 24hrs, as I said they are super keen. I dont know of anywhere else, sorry. J x